The present application is directed to a process and apparatus for treating water containing bromides with ozone so as to minimize bromate formation from the bromides by the ozone.
Water from lakes and rives must be disinfected before it is considered acceptable as drinking water or delivered to distribution systems. Different disinfectants, including chlorine, have been used for this purpose and ozone is often preferred because it is highly effective, it is relatively quick acting, it produces better tasting water than some other disinfectants and it is readily available. However, ozone produces an undesirable chemical reaction with bromides.
In particular, certain water have significant concentration of various bromides (salt resulting from a reaction with hydrobromic acid wherein bromine is in a negative one oxidation state: Brxe2x88x92). Ozone converts at least some of these bromides to bromates (a salt of bromic acid: BrO3xe2x88x92). In the past such bromates have not been closely monitored or regulated, but recent U.S. governmental regulations have placed limits on bromates at 10 micrograms per liter of water. There is reason to believe that the acceptable level of bromates will be further reduced to at least 5 micrograms per liter of water.
Direct ozonation of certain fresh water sources at a level producing acceptable disinfection in the water, produces greater levels of bromates than are currently acceptable or will be acceptable in the future.
Since ozone is often a preferred disinfectant, various ways have been developed to try to reduce the bromate formation.
One method to reduce bromate formation is by the addition of ammonia to the water prior to ozonation. This has some beneficial effect, but produces additional problems. Most water systems add some chlorine to the water subsequent to treatment by ozone, as such chlorine remains active in the water longer than the ozone and functions to help maintain the distribution system free of bacteria or other microorganisms after the water leaves the treatment facility. When ammonia is used prior to ozonation to reduce bromate formation, there is substantial residual ammonia in the post ozonation water and this residual ammonia must be neutralized by the chlorine before a free or residual level of chlorine can subsequently form in the water with chlorine addition. As chlorine must be added in a ratio of about 10 to 1 by weight to neutralize the ammonia, the ammonia in the water results in a large chlorine addition and substantial cost. Alternatively, the ammonia can be removed by reaction with microorganisms or physically by air stripping, but both can present problems when large quantities of ammonia must be used in conjunction with comparatively large amounts of ozone.
A second method of reducing bromate production that has been previously used is the reducing of the water pH to less than 6.5 so as to interfere with certain reactions that produce bromates. A major problem of pH control of this type is that substantial quantities of acids must be used to lower the pH and later stoichiometric quantities of bases must be added to the water to return the water to a neutral pH after ozonation. Such a process is both expensive and adds a substantial amount of dissolved solids to the finished water.
A water treatment process is provided wherein the water initially includes bromides, the water is to be at least partially disinfected with ozone and it is desirable to minimize or reduce the formations of bromates from the bromides as compared to such a process utilizing ozone above and without taking other steps to reduce bromates.
In the process of the invention water is pretreated with chlorine prior to ozonation. The use of chlorine in this way reduces the chemical load in the water requiring oxidation, since the chlorine is effective in oxidizing certain but not all oxidizable components of the water in a relatively short time. This results in less of a chemical load requiring oxidation by ozone, such that substantially less ozone is required to complete oxidation and substantial disinfection of the water, as disinfection results from oxidation of microbes in the water.
Any significant amount of chlorine added as a pretreatment to ozonation reduces the ozone requirement until such point as the chlorine is no longer an effective oxidant. Preferably, the chlorine is added until chlorine demand is met whereat substantially all chorine added prior to the chlorine demand being met is utilized in an oxidation process and subsequent addition of chlorine after the demand is met results in free or residual chlorine remaining in the water.
If chlorine is added past the noted breakpoint, then it is desirable to provide substantial residence and reaction time before ozonation to allow the remaining chlorine to either react with components of the water or to dissipate to a non-detectable or trace amounts. It is important that the residual chlorine level be totally reduced to zero or at least minimized before ozonation, since the free chlorine will react with and waste the ozone, so as to increase the ozone demand. Secondly, the free chlorine can be stripped from the water by the ozone gas and carried into ozone destruct units wherein the chlorine can foul the catalyst used for ozone destruction. Furthermore, if ammonia is utilized, residual chlorine reacts with the ammonia to produce chloramines and reduce the efficacy of the ammonia in reducing bromates.
Therefore, while chlorine can be added prior to ozonation below, at or above the noted breakpoint in accordance with the invention, it is preferred that it be added at or quite close to the breakpoint. The chlorine, especially when used at or below the breakpoint level, results in a reduction in the subsequent ozone dosage required to achieve the same ozone residual and decay rate for disinfection and also a reduction in bromate formation.
It has also been found that the pretreatment with chlorine can be used advantageously and synergistically with pretreatment with ammonia to further reduce the bromate formation resulting from ozone treatment. It is believed that the chlorine reduces the ozone required which in turn reduces the bromates formed. It is also believed that the ammonia interacts with various bromides in the water so as to block reaction with ozone and subsequent conversion of such bromides to bromates. The combination of pretreatment with chlorine and ammonia has been found to work synergistically so as to significantly reduce bromate formation in comparison to water treated only with ozone to produce the same degree of disinfection.
When chlorine and ammonia are used together as pretreatments, it is preferred that the chlorine be added first and then the ammonia. It is also preferred, as noted above, that the chlorine be added at or below the breakpoint prior to any addition of ammonia or alternatively that the chlorine be allowed to react over a relatively long time period or otherwise be dissipated prior to addition of ammonia so that free chlorine does not significantly react with the ammonia and thereby reduce the ability of the ammonia to interfere with production of bromates during ozonation. It is preferred that ammonia be present in the water before and especially during ozonation.
Subsequent to initial chlorine addition at or below the breakpoint or after dissipation of the chlorine, ammonia is injected into the water. Normally, ammonia addition is in the range form 0.01 to 5.0 milligrams per liter of water. Preferably, the ammonia addition would be in the range from 0.1 to 0.5 milligrams per liter with 0.1 milligrams per liter being most preferred with many water supplies; however, the preferred ammonia treatment will depend by choice somewhat on the bromide level in the raw water supply and the ozone required for disinfection. With some of the U.S. water supplies, a treatment with 0.1 to 0.3 milligrams per liter of water of ammonia subsequent to chlorination to the breakpoint and subsequent ozonation, results in a bromate concentration of less than 5 micrograms of bromates per liter of water.
Subsequent to chlorination and injection of ammonia (where used), ozone is added in an amount sufficient to achieve desired levels of disinfection. The quantity of ozone injected into the water under the present invention (whether with just chlorine pretreatment or with chlorine and ammonia pretreatment) is less than is required using just ozone to achieve the same degree of disinfection. In some cases a higher ozone concentration could be used to oxidate and disinfect oxidizable components of the water. The combination of these pretreatments results in lower bromate concentration in the resulting water.
Subsequent to ozonation, residual ozone may be allowed to react and dissipate or may be removed by stripping or the like.
The ozonation process normally produces a substantial quantity of small or short chained organic molecules, such as aldehydes and carboxylic acids, as well as other microorganism assimiable organic carbon or molecules, because the ozonation process breaks some larger molecules (that cannot be assimilated by microorganisms) into such smaller molecules. These smaller molecules are undesirable in the final water supply because such can function as food for microorganisms in the distribution system leading to growth and dispersion of such microorganisms, especially those intended to be destroyed by ozonation. Consequently, it is preferred to reduce or eliminate such molecules. For this reason the water after ozonation is preferably then filtered. The filter may be only an activated carbon filter designed to absorb small organic (and microorganism edible) molecules.
However, preferably, the filters are provided with a matrix of biota (microbes or microorganisms) that effectively assimilate the small organic and edible molecules and produce nonedible by-products. Such biota is known in the art of waste and fresh water treatments and would preferably be an aerobic bacteria.
Furthermore, free ammonia may still be present in the water subsequent to ozonation and additional biota may be provided at such a filter to convert remaining ammonia to nitrites and then further to nitrates, such as is provided by Nitrosomonas and Nitrobacter respectively.
After dissipation of the ozone subsequent to ozonation, it is often desirable to add additional chlorine to maintain a reduced level of or completely prevent microorganism growth in the water in a distribution system subsequent to treatment under the process of the invention. Preferably, such chlorine is added subsequent to passage of the water through a filter of the type described above and, especially subsequent to removal of free ammonia from the water so as to reduce the quantity of chlorine needed to provide a desired free chlorine level in the water. Ammonia is removed prior to the second addition of chlorine, as the ammonia would otherwise react with the chlorine. Preferably, chlorine is added after ozonation above a breakpoint for the chlorine in the water, so that there is free or residual chlorine present in the water. The level of residual chlorine depends on the distribution system and the desires of the operators of that system; however, a level of 1.5 milligrams of chlorine per liter of water passing into the distribution system has been found to be an effective amount in many facilities. The chlorine also functions to further disinfect the water subsequent to the filter to remove microorganisms that may enter the water at the filter.
In summary it is desired to provide a facility and method of treatment of water so as to disinfect such water at least partially with ozone while minimizing production of bromates from bromides in the water and while maintaining at a minimum the use of other chemicals in the treatment of the water.
Therefore the objects of the present invention are: to provide a process for disinfecting water including bromides with ozone so as to reduce the production of bromates from the bromides; to provide such a process wherein chlorine is used to pretreat the water prior to ozonation; to provide such a process wherein ammonia is used to treat the water subsequent to the pretreatment with chlorine and before the treatment with ozone; to provide such a process wherein the water is treated by filtration and thereafter chlorination subsequent to ozonation; to provide facilities for such a process; and to provide such a process that is easy to operate, utilizes readily available chemicals, is comparatively inexpensive to operate and is especially well suited for the intended use thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.